1. Field of the Invention
This invention relates to microreactors and more particularly to microreactors for reforming hydrocarbon fuels and generating hydrogen gas.
2. Description of the Related Art
A microreactor (or microstructured reactor or microchannel reactor) is a device in which chemical reactions are designed to take place in confined spaces having lateral dimensions of less than 1 mm. Currently, there are major technological issues that prevent current technology from meeting the needs of microreactors for generating hydrogen, syngas, or performing specialty chemical synthesis. In general, gas-phase reactions for generating hydrogen and other specialty chemicals require microfabricated components that can perform under harsh operating conditions such as high temperatures, high temperature transients, or corrosive or erosive environments.
Current microfabrication processes (e.g., wet etching, dry etching, lithography, LIGA, etc.) are primarily applicable to silicon, photoresists, and metals. These materials readily corrode when subjected to hot gas streams that contain corrosive ingredients such as oxygen, steam, CO2, sulphur, and trace metals, each of which may be present when generating H2 from natural gas or using gas-phase specialty chemical synthesis. Another important factor when generating hydrogen gas for fuel cells is the H2 to CO ratio, since a higher ratio reduces the cost of CO removal.
The few processes that are available for microfabricating ceramics are either prohibitively expensive due to the need for very expensive precursor materials, such as pre-ceramic polymers, or are unable to attain the high precision required as a result of shrinkage that occurs during sintering. The shrinkage problem in particular often creates a need for very expensive secondary machining operations. Furthermore, many high temperature gas-phase reactions require passing gas streams over catalysts. The introduction of catalysts into silicon, metal, or sintered-ceramic-based microreactors is typically a complex multi-step process that requires a high surface area (i.e., highly porous) wash-coat to be applied inside the channels of the microreactor prior to catalyst deposition. This wash-coat is necessitated by the very low component surface area of silicon and metals. The wash-coat is typically easily damaged during operation because it poorly bonds with silicon, metal or sintered ceramic. This characteristic undesirably shortens the life of the microreactor.
In view of the foregoing, what are needed are robust ceramic materials for fabricating microreactors that are able to withstand high temperatures, high temperature transients, or corrosive or erosive environments and thus have excellent thermal shock resistance and thermal cycling properties. Ideally, such materials would enable features to be fabricated in net-shape and net-size with very high precision. Further needed are microreactors that increase the H2 to CO ratio to reduce the cost of CO removal by secondary operations such as membrane reactors. Further needed are microreactors to increase the amount of hydrogen that can be extracted from hydrocarbon feedstock fuels. Further needed are ceramic materials that enable cost-effective fabrication of microreactors. Yet further needed are porous ceramic materials with intrinsically high surface area that can be infiltrated with catalysts to increase hydrocarbon reformation efficiency.